1. Field of the Invention
This invention relates to anisotropic plasma etching of silicon.
2. Description of the Prior Art
Plasma etch processes currently used to etch materials for semiconductor device fabrication consist of an electrical discharge of halogen bearing gases. Halogens typically encountered in these processes are fluorine, chlorine and bromine. The process begins with application of a masking material, such as photoresist, to protect the desired geometries of the device from the etch process. The device in process is then placed in a plasma reactor and etched. The subsequent steps are determined by the type of device being fabricated. This process is especially valuable for the definition of small geometries on the order of one to five microns. For definition of geometries of less than one micron, it is essential that the etching proceed only in the vertical direction. The fragile nature of the small geometry structures cannot have a reasonable degree of reliability if any significant amount of undercutting takes place during the process.
A very common silicon etch process is based on fluorine. When mixtures such as CF.sub.4 -O.sub.2 are dissociated in an electrical discharge, fluorine atoms are liberated, and volatilize the silicon as SiF.sub.4. Such processes are isotropic; they etch in all directions at the same rate. Anisotropic, or vertical, etches in silicon are not observed when fluorine is the sole etchant.
In U.S. Pat. No. 4,226,665, Mogab describes etch chemistries which yield vertical etches. For vertical etching of silicon a chemistry such as C.sub.2 F.sub.6 -Cl.sub.2 is indicated. The C.sub.2 F.sub.6 serves as a source of "recombinants", such as CF.sub.3. The recombinants suppress etching in the horizontal direction by recombining with Cl atoms, which have been adsorbed on the etched walls. Etching can proceed in the vertical direction because ion bombardment from the plasma suppresses the recombination mechanism.
Chemistries based on chlorine have been considered to be necessary for vertical etching of silicon, and discharges of pure Cl.sub.2 have been found useful for this purpose. However, some silicon materials, such as highly doped polysilicon, still experience some undercutting if etch conditions are not closely controlled.
Furthermore, the use of chlorocarbons as the Cl source for etching polysilicon is not desirable. Selectivity to the underlying SiO.sub.2 layer is a constant problem because the C is a good reducer of SiO.sub.2, combining with the O to form CO and CO.sub.2. A practical limit on the selectivity of doped polysilicon to SiO.sub.2 appears to be roughly 20:1.
To improve upon this prior art, a plasma etch chemistry was developed which provides a near perfect anisotropic etch of silicon wherein a Cl containing compound such as BCl.sub.3 with Br.sub.2 added thereto is used.
However, bromine has disadvantages as an additive reagent. Bromine is a liquid source with a vapor pressure of 150 torr. at room temperature. Bromine tends to condense in the coolest section of the line. Heating of the line is required to eliminate condensation, particularly in the metering valves that regulate the flow.